1. Field of the Invention
Aspects of the present invention relate to an activation method and system capable of securing the uniform performance of a laminated fuel cell stack.
2. Description of Related Art
A fuel cell is a system that directly converts various fuels, such as natural gas, liquefied natural gas, kerosene, coal, naphtha, methanol, hydride, and waste gas, into electricity. The fuel cell has come into the spotlight as a next-generation, clean, power generation system. A fuel cell may be a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), a polymer electrolyte fuel cell (PEFC), a phosphoric acid fuel cell (PAFC), an alkaline fuel cell (AFC), and the like, depending on an electrolyte used therein.
A polymer electrolyte membrane fuel cell and a proton exchange membrane fuel cell (PEMFC) use an ion-exchange membrane that includes a solid polymer electrolyte. The solid polymer electrolyte is resistant to corrosion and evaporation, and allows for a high current density per unit area. Further, since a PEMFC has a higher output and a lower operation temperature than other fuel cells, PEMFCs are often used to power small electronic devices, such as notebook computers. Furthermore, research has been actively conducted to develop a PEMFC to power automobiles, yachts, buildings, or the like.
A direct methanol fuel cell uses an ion-exchange membrane that is made of a solid polymer electrolyte. The direct methanol fuel cell is typically operated at a temperature below 100° C., using a liquid fuel such as methanol instead of a fuel reformer. For this reason, a direct methanol fuel cell is suitable as a power source for small electronic devices.
A direct methanol fuel cell may be manufactured as a laminated stack or a flat plate stack. The laminated stack includes a plurality of cells that are laminated to one another, by interposing separators between the cells. The flat plate stack includes a plurality of cells arranged in the same plane, and electrically coupled in parallel.
Each cell of a direct methanol fuel cell stack includes an anode, a cathode, and an electrolyte disposed therebetween. Each cell generates a voltage of about 0.4 to 0.6V. When an output of about 5V is required, a fuel cell stack may include at least 10 cells, which are electrically coupled in series.
An activation process is performed on a fuel cell stack, after the fuel cell stack is assembled. The activation process includes initially operating a fuel cell stack under a specific load, for a predetermined time, to check the maximum performance thereof, before the fuel cell stack is delivered. Generally, a high-concentration polar solvent is used to reduce the length of the activation process. For example, it takes about 1 hour to activate a laminated fuel cell stack, using a 10 molar methanol aqueous solution. On the other hand, it takes about 70 to 100 hours to activate a laminated fuel cell stack, using a 1 molar methanol aqueous solution.
After the activation process is performed, an initial performance test is generally performed. In the initial performance test, a desired performance may not be obtained from a specific cell in a fuel cell stack, due to certain defects. Such defects may occur when a membrane electrode assembly (MEA) is defective, or when a fuel flow channel in a separator is blocked, by an impurity introduced during manufacturing or introduced from a fuel. In most cases, the impurity can be easily removed from the fuel flow channel, by increasing a flux of the fuel, or by reversing a flow direction of the fuel.
Generally, after an impurity is removed from the channel of a defective cell, the defective cell still does not show a desired performance. This is because the initial activation process was not appropriately performed, due to the impurity.
When the activation process is repeated to activate the aforementioned defective cell, the non-defective cells are also reactivated. The non-defective cells can be degraded by the reactivation. Therefore, only the defective cells should be activated, without reactivating the non-defective cell. Conventionally, this is accomplished by disassembling the fuel cell stack, so as to activate only the defective cell. However, disassembling a laminated stack is complicated and increases processing time.
In addition, in activated cells, an electrolyte membrane is expanded. Therefore, it is difficult to properly disassemble and reassemble a laminated stack. Further, this process may lead to a decrease in the performance of the laminated stack.